THE ASTROPHYSICAL JOURNAL, 490:328, 1997 November 20
THE "MASS DISCREPANCY" FOR MASSIVE STARS: TESTS OF MODELS USING
SPECTROSCOPIC BINARIES
VALORIE BURKHOLDER
Department of Physics and Astronomy, Northern Arizona University,
P. O. Box 6010, Flagstaff, AZ 86011-6010; veburkho@mtholyoke.edu
PHILIP MASSEY
Kitt Peak National Observatory, National Optical Astronomy Observatories,
P. O. Box 26732, Tucson, AZ 85726-6732; massey@noao.edu
AND
NIDIA MORRELL
Facultad de Ciencias Astronomicas y Geofisicas, Universidad Nacional
de La Plata, Argentina; nidia@fcaglp.fcaglp.unlp.edu.ar
ABSTRACT
Stellar evolutionary models are often used to infer a star's mass
via its luminosity, but empirical checks on the accuracy of the
theoretical mass-luminosity relation for very massive stars have been
lacking. This is of particular concern given that modern atmosphere
models yield systematically smaller masses for massive stars than do
evolutionary models, with the discrepancy a factor of two for Of
stars. We attempt to resolve this mass discrepancy by obtaining new,
high-resolution optical data on seven early-type spectroscopic
binaries: V453 Cyg, HD 191201, V382 Cyg, Y Cyg, HD 206267, DH Cep, and
AH Cep. Our study produces improved spectral substypes for the
components of these systems, crucial for evaluating their luminosities
and locations in the H-R diagram. Our radial velocity study utilizes
a measuring method which explicitly accounts for the effects of
pair-blending. We combine our new orbit solutions with existing data
on inclinations and distances when available to compare the orbital
masses with evolutionary models, and we find good agreement in all
cases where the stars are non-interacting. (The components of V382 Cyg
and DH Cep fill their Roche lobes, and in both cases we find masses
substantially lower than the masses inferred from evolutionary tracks,
suggesting that significant material has been lost rather than
transferred. We confirm that this same trend exists for other systems
drawn from the literature.) Our own data extends to only 15 Msun,
although photometric inclination determinations for HD 191201 and HD
206267 should prove possible, and will provide examples of higher mass
systems. We briefly discuss suitable systems from the literature, and
conclude that orbit solutions provide good agreement with the
evolutionary models to 25 Msun. Beyond this, most known binaries
either fill their Roche lobes or have other complications. We also
discuss five systems for which our improved data and analysis failed
to yield acceptable orbit solutions: EO Aur, IU Aur, V640 Mon
(Plaskett's star), LY Aur, and 29 UW CMa all remained intractable,
despite improved data.
Subject headings: binaries: spectroscopic -- stars: early-type --
stars: evolution -- stars: fundamental parameters